1. Field of the Invention
This invention relates generally to methods of forming a read sensor for a magnetic head using a lift-off mask, and more particularly to a method of forming a read sensor for a magnetic head using a lift-off mask which includes a hardmask layer and a release layer.
2. Description of the Related Art
Magnetic head assemblies are typically made of multiple thin film layers which are patterned to form various shaped layers in the head. Some of the layers are plated while other layers are sputter deposited on a wafer substrate. The read head portion of a magnetic head assembly includes multiple layers that are typically sputter deposited. For example, the multiple layers of a read sensor, hard bias and lead layers connected to the read sensor, and first and second read gap layers below and on top of the read sensor are typically sputter deposited.
One prior art method of forming shaped sputter deposited layers is to sputter deposit a full film layer of the required material on a wafer substrate, form a patterned photoresist layer on the layer, ion mill away the exposed portion of the layer and then remove the photoresist layer leaving the desired shaped layer that was protected therebelow. This first conventional method of shaping sputter deposited layers has been generally superseded by a second conventional method which utilizes a bilayer lift-off mask scheme.
The bilayer lift-off mask used in the second conventional method has a T-shape (as seen in cross-section) wherein the vertical portion of the T is short and wide but less wide than the horizontal top portion of the T. The top portion of the T is generally a patterned photoresist layer and the bottom vertical portion of the T is a release layer. This configuration provides first and second undercuts (as seen in cross-section) wherein each undercut has a height and a length below the top photoresist portion.
In this method, the bilayer lift-off mask is employed for the purpose of making contiguous junctions of the first and second lead layers with first and second side edges respectively of the read sensor. Multiple read sensor layers are sputter deposited in full film on the wafer substrate followed by formation of the bilayer lift-off mask covering a read sensor site. Ion milling is then employed to remove all of the read sensor material except that below the mask. Full films of hard bias and lead layer materials are then sputter deposited which cover the top of the lift-off mask and an area surrounding the lift-off mask. It is important that the height and length of the undercuts is sufficient such that a photoresist stripper can reach the bottom release layer. The stripper is then introduced to dissolve the bottom release layer after the hard bias and lead layer depositions. This causes the bilayer lift-off mask and the hard bias and lead materials deposited thereon to be released from the wafer substrate leaving the aforementioned contiguous junctions between the first and second lead layers and the first and second side edges respectively of the read sensor.
The bilayer lift-off mask scheme significantly improves the making of read heads by forming contiguous junctions between the lead layers and the read sensor. Fewer processing steps are required and the profile of the lead and hard bias layers above the read sensor has been reduced. Unfortunately, however, many bilayer lift-off masks using this conventional methodology are better suited for the construction of read heads with a track width of greater than approximately 0.2 microns. The more narrow the track width, the greater the tracks per inch (TPI) that can be read by the read head from a rotating magnetic disk. Accordingly, the greater the tracks per inch the greater the storage capacity of a disk drive employing such a read head.
Using such conventional methods, processing control of the length and height of the undercut has not been precise enough for very narrow track widths. Long first and second undercuts leaves insufficient release layer material which causes the bilayer lift-off mask to be separated from the substrate or topple over during subsequent processing steps of ion milling and sputter deposition. If the undercut is too short, fencing can occur. Fencing is the deposition of sputtered material across the height of the undercut that remains after the photoresist is removed. These particular problems have generally been caused by photoresist developers of strong normality which are employed to pattern the release layer. Because of the rapid removal of the release layer portion by photoresist developers of high normality, such as tetramethylammonium hydroxide (TMAH), 0.26N, or aqueous potassium hydroxide (KOH), 0.3-0.4N, it has been difficult to precisely stop the removal of the release layer portions forming the aforementioned undercuts.
Several other problems exist in forming read sensors using these methods. Typical processing of prior art bilayer lift-off masks has been to treat a single layer of photoresist with ultraviolet or an electron beam to a particular depth. Unfortunately, the penetration depth of the beam has not been precise enough to form a highly-defined bottom release layer portion of a single resist layer. The height control is important for a successful lift-off process for a given hard bias and lead thickness. Still a further problem with the present processing of bilayer lift-off masks is that the ion milling step reduces the width of the top photoresist layer portion. This reduction undesirably reduces the track width of the read head in an uncontrolled manner.
Variations of the lift-off mask scheme have improved the formation of the read sensor and solved some of these problems to a limited extent. In U.S. Pat. No. 6,218,056 B1, for example, the lift-off mask is subjected to an electron beam for decreasing the molecular weight of the release layer and increasing the weight of the photoresist layer so that a weak developer can be employed for patterning the release layer which does not alter the track width of the photoresist layer for better control.
Using any one of the above-processes, however, the existence of a photoresist layer in the lift-off mask during the ion milling and deposition processes places a limit on how narrow the track width of the read sensor can be. Commercially available photoresists are thicker than 3000 Angstroms. Unfortunately, in such cases, a thick top photoresist layer causes shadowing where a read sensor having relatively long side edges with gradual slopes is formed. However, a preferred read sensor has short side edges with steep slopes for a narrow track width. The widths of today's magnetic heads are approaching 0.1-0.2 μm.
Accordingly, there is a strong-felt need for a method of forming a read sensor using a lift-off mask that is thin and sufficiently shaped so that the read sensor can have a narrow track width.